1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the zoom lens. In particular, the present invention relates to a zoom lens useful for a digital camera, a video camera, a television (TV) camera, a monitoring camera, a silver-halide film camera, and the like.
2. Related Background Art
A photographic lens used in an image pickup apparatus, such as a digital camera and a video camera, generally includes a zoom lens. According to current market preferences, it is desired a zoom lens having a wide angle of view and a high zoom ratio that can offer high optical performance for the entire zoom range.
As a zoom lens having a high zoom ratio, a positive-lead type zoom lens has been widely used. In the positive-lead type zoom lens, a lens unit having a positive refractive power is positioned closest to an object to be imaged. U.S. Pat. No. 6,594,087 discusses a zoom lens that uses an anomalous dispersion material as a material of a lens constituting a first lens unit and having a zoom ratio of about 10. In addition, U.S. Pat. No. 7,304,805 discusses a five-unit zoom lens including, in order from the object side to the image side, positive, negative, positive, negative, and positive lens units, in which a lens of the first lens unit is made of an anomalous dispersion material.
In a positive-lead type zoom lens, it is relatively easy to achieve a high zoom ratio while reducing the total size of the zoom lens. However, if the zoom ratio and the angle of view are increased and expanded in the positive-lead type zoom lens, the secondary spectrum of chromatic aberration of magnification and that of axial chromatic aberration may increase in the zoom range closer to a wide-angle end and in the zoom range closer to a telephoto end, respectively.
In order to achieve a high optical performance for the entire zoom range while achieving a high zoom ratio and a wide angle of view at the same time in the positive-lead type zoom lens, it is significant to reduce chromatic aberration, in particular, the secondary spectrum thereof. In order to reduce chromatic aberration, a low-dispersion lens made of an anomalous dispersion material can be provided and used at an appropriate location in the zoom lens.
In addition, in order to reduce chromatic aberration, it is significant to provide an optimal refractive power to each lens unit constituting the zoom lens based on the characteristics (an Abbe number and a relative partial dispersion) of the material of the lens. In particular, in the above-described positive-lead type five-unit zoom lens, an axial ray passes through the first lens unit having a positive refractive power at the highest vertical level.
Accordingly, if a high zoom ratio is desired, it becomes difficult to reduce the secondary spectrum occurring at the telephoto end unless the material having the appropriate characteristics is used as a material of the lens constituting the first lens unit. If the amount of the secondary spectrum is large, it becomes difficult to achieve a high optical performance for the entire zoom range.